I have an area in the 1940? section of my home where due to water seepage around a replacement window I have extensive termite damage. I have a 3' plus area where the 6"x8" sill beam has been completely eaten away. The nasty bugs did do eat the floor joists just the sill beam the joists were notched into. The joists are sitting on the brick foundation and the floor is not sagging. I am going to have to replace some wall studs and will frame a new opening and replace the window to get rid of the leak. Wondering if I can use concrete to replace the sill beam and give a solid place for the wall studsto sit?

You can but you may be just moving the problem. You don't want wood touching concrete, if you just change wood for wood you use a sill gasket under it. If you replace it with concrete you have to also protect the joists from the concrete. If you don't have 6 to 8 inches of foundation showing on the outside, concrete may be a good idea.

That is part of my issue, I don't have enough cleance to the ground so it is very inviting for the bugs to find the wood. I have dung further and I find I have about 6 to 8' which I need to replace. I may try to come from the outside by pulling siding and 1x12 sheeting. Then I will attempt to get 2x8 treated at least doubled into the space to support wall studs. I am not really sure why they are still standing at this point. Of course there are yews right up against the house. But I guess if it was easy everyone would do it.

Usually what you find is a 2x sill plate with a single rim joist, with the joists also sitting on the sill plate and five nails thru the rim into each joist and "now" you would have an extra block under each side of the window below the jack studs.
So however you do it would be fine as long as you have some way to attach everthing back together so that nothing can move or warp. Treated lumber is ok as it is outside the living space of the house. You should remove the trees and reshape the yard but you know that.

Yes in conventional modern contruction that would be the case. This section is old enough that the 6x8 sill beam had notches for the floor joists to sit in and the studs are nailed to the sill as well. The sill rests on the 6" side and rises 8" up from the foundation. The floor stops at the wall studs and does not go under them. Basically balloon contruction. I am hoping by using a double 2x on edge I will have enough support under the wall studs and can attach the floor joists by shiming between them and the brick and using hanger brackets to support them as well. I would really like to jack everything up and start over but I have run out of decent weather.

Either extending the oundation upwards, your "concrete sill plate, or replacing the sill with PT 2x's is gonna work. Be sure to use proper joist hangers that support bottoms of joists and "fill every hole" a hanger nail or screw in every hole in the hanger. Note thatholes that are manufacturing holes" that are not filled, why they need holes to make a hanger, I dunno, but there they are. Those holes are either labelled on the hanger, a different shape, square or diamond or such, or are identified on the tech sheet you get where you buy hangers.

You'll also want a water barrier between concrete and wood, neal's gasket. When I was starting out, we still set plates in a bed of mortar on the foundation, not only to seal the gap but also to help in leveling the plate. Then we mainly used asphalt or tar. Nowadays there's all sorts of products, some in a tube applied with caulking gun, some rolls of “rubber,” some that are also insulation. I think it would be a real good idea to install metal termite shields between cocrete and wood. Metal flashing that extends a couple of inches on each side of foundation, slanted down at 45 deg angle. Some of that comes with a peel and stick rubber “gasket on each side. You can just sandwich flat flashing between foundation, goopy, flashing goopy, and plate, and bend the outer edges down.

Wouldn't be a bad idea to rip off more siding along length and check for termite damage farther along. Damage may not show, poke with screw driver or ice pick or such.

Ice pick? Back when I was setting sills in mortar we got ice picks free at the ice house. Now ya gotta search the hardware store, they aren't in the kitchen area with cast iron pans and pressure cooker gaskets, now they are in tool area. If ya ask a younger clerk where they are, he'll stare at you blankly. So ask the old guy in the back. The one that's at least as old as me.

__________________

Measure twice, cut once.

Look at the nail, not the hammer. Watch the fence, not the blade.

If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.

All in all, I'd say you're correct, neal, but just the other day i was over in a real hardware store and right there in the tool section was an old fashioned ice pick, plainly labeled on the display card as such. wasn't any advertising on the handle though. Now it was hanging amongst the awls, the scratch awl, the screw awl, the leather awl, and i dunno what all awls, there was a awful lot of awls all hangin there. I got a leather awl called "The Awl for All" with a spool of waxed thread built in and 3 difurnt needles in the handle. As handy as it is, Fireguy, don't try to sew the joists to the sill.

__________________

Measure twice, cut once.

Look at the nail, not the hammer. Watch the fence, not the blade.

If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.